1. Technical Field of the Invention
The present invention relates generally to a fuel injector to be installed in an internal combustion engine to spray fuel thereinto, and more particularly to such a fuel injector which has installed therein a fuel pressure sensor, a sensor circuit, and an electric shield which works to shield the sensor circuit from electric noises arising from drive terminals which are used to control an operation of the fuel pressure sensor.
2. Background Art
In order to ensure the accuracy in controlling output torque of internal combustion engines and the quantity of exhaust emissions therefrom, it is essential to control a fuel injection mode such as the quantity of fuel to be sprayed from a fuel injector or the injection timing at which the fuel injector starts to spray the fuel. For controlling such a fuel injection mode, there have been proposed techniques for monitoring a change in pressure of the fuel upon spraying thereof from the fuel injector.
Specifically, the time when the pressure of the fuel begins to drop due to the spraying thereof from the fuel injector may be used to determine an actual injection timing at which the fuel has been sprayed actually. The amount of drop in pressure of the fuel arising from the spraying thereof may be used to determine the quantity of fuel actually sprayed from the fuel injector. Such actual observation of the fuel injection mode ensures the desired accuracy in controlling the fuel injection mode.
For instance, in the case where a change in pressure of the fuel arising from the spraying of the fuel from the fuel injector (which will also be referred to as a fuel pressure change below) is measured using a pressure sensor installed directly in a common rail (i.e., a fuel accumulator), it will be somewhat absorbed within the common rail, thus resulting in a decrease in accuracy in determining such a fuel pressure change. In order to alleviate this drawback, Japanese Patent First Publication No. 2000-265892 teaches installation of the pressure sensor in a joint between the common rail and a high-pressure pipe through which the fuel is delivered from the common rail to the fuel injector to measure the fuel pressure change before it is absorbed within the common rail.
The fuel pressure change, as produced at a spray hole of the fuel injector through which the fuel has been sprayed, will, however, surely attenuate within the high-pressure pipe. The use of the pressure sensor installed in the joint between the common rail and the high-pressure pipe, therefore, does not ensure the desired accuracy in determining the fuel pressure change. The inventors have study the installation of the pressure sensor in a portion of the fuel injector which is located downstream of the high-pressure pipe. Such installation, however, has been found to pose the problem, as discussed below.
The pressure sensor is typically made up of a body in which a high-pressure path is formed to supply high-pressure fuel to a spray hole, a valve actuator installed in the body to move a valve to open or close the spray hole, drive terminals to supply electric power to the valve actuator. The power is supplied or blocked to the valve actuator to turn on or off frequently, so that the drive terminals will radiate electric noises. The fuel pressure sensor also has installed therein an amplifier working to amplify an output thereof, which will be, however, exposed undesirably to the noises arising from the drive terminals.